Self-recoverable covering material and method for fabricating the same

ABSTRACT

The surface of a member formed of at least one kind of ceramics, metals and C/C composites, is covered with an outermost surface oxide covering layer having a stability against the environment in the atmospheric air at 1500° C.; and a self-recoverable layer as an intermediate layer formed between the outermost surface oxide covering layer and the surface of the member. The self-recoverable layer has an oxidizing rate greater than that of the outermost surface covering layer for recovering cracks generated on the outermost surface covering layer, and a compound produced by the self-recoverable layer has the same phase as in the outermost surface covering layer. To achieve the optimum self-recovering function at high temperatures, the outermost surface oxide covering layer may be made of a stable oxide having a vapor pressure of 1×10 -4  /m 2  or less in the atmospheric air at 1500° C., and the oxidized product material from the self-recoverable layer contains an oxide having the same composition as that of the outermost surface oxide covering layer.

This is a division of patent application Ser. No. 08/309,149, filed Sep. 20, 1994, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a process and composition of matter capable of recovering cracks generated in mechanical structural components.

When metals, carbon/carbon-fiber composites (hereinafter, referred to as "C/C composite") and non-oxide ceramics are used in high temperature oxidizing atmospheres, the surfaces thereof are oxidized. To prevent the oxidization, the surfaces of the above materials are covered with isolating layers having protective materials therein (hereafter sometimes referred to as "PMs", for example, zirconium dioxide). For example, a method of covering zirconium dioxide as an isolating layer on the surface of a metal member is disclosed in Unexamined Japanese Patent Publication Nos. SHO 55-28351 and SHO 58-87273. A method of covering a carbon member with zirconium dioxide as an isolating material is disclosed in Unexamined Japanese Patent Publication No. SHO 58-125679; and a method of covering a silicon nitride member with zirconium dioxide as an isolating layer is disclosed in Examined Japanese Patent Publication No. HEI 5-8152. Further, it is known to cover a cutting tool with a boride and then with alumina for increasing the adhesiveness of the covering.

In non-oxide ceramics, it is also known that they are heated in the atmospheric air at high temperatures to recover voids and cracks generated on the surface.

The above-described references, however, do not examine the reliability under the actual service environment. For example, if a base material is covered with an oxide, such as zirconium dioxide, at the time when the covering layer is cracked, due to its inherently brittle nature, the base material, having a poor resistance to an environmental reactive material (here, oxygen), is exposed and is rapidly oxidized. The base material is oxidized by the generation of cracks, resulting in reduced mechanical strength.

Accordingly, there is a need for an isolating layer which when used will resist reaction of a base material with an environmentally reactive material (hereafter sometimes referred to as "ERM") when the isolating layer is cracked. An object of the present invention, therefore, is to provide a self-recoverable isolating layer having a reactive material (hereafter sometimes referred to as an "RM") which, when exposed to an environmental reactive material ("ERM") reacts with the ERM to recover cracks generated on an outermost surface of the isolating layer.

SUMMARY OF THE INVENTION

To achieve the above object, according to an embodiment of the present invention, the surface of a member formed of a base material (for example: ceramics, metals and/or C/C composites) is covered with an isolating layer having a reactive material therein, wherein the reactive material reacts upon exposure to the environmental reactive material to form a protective material.

According to a more specific embodiment, the isolating layer comprises a reactive material and a protective material, distributed such that there is more protective material near the environmental reactive material and more reactive material near the base material. According to still a more specific embodiment, the isolating layer comprises two distinct layers, wherein the layer closest to the environmental reactive material comprises a protective material and the layer closest to the base material comprises a reactive material.

In a more specific embodiment of the present invention, the surface of a member formed of at least one kind of ceramics, metals and C/C composites, is covered with an outermost surface oxide protective layer having a stability against the environment in the atmospheric air at 1500° C., and a self-recoverable layer of reactive material formed between the outermost surface oxide covering layer and the surface of the member. According to one such embodiment, the self-recoverable layer has an oxidizing rate greater than that of the outermost surface covering layer for recovering cracks generated on the outermost surface covering layer, and a compound produced by the self-recoverable layer has the same phase as in the outermost surface covering layer. To achieve the optimum self-recovering function at high temperatures in one such embodiment, the outermost surface oxide covering layer comprises a stable oxide having a vapor pressure of 1×10⁻⁴ N/m² or less in the atmospheric air at 1500° C., and the oxidized product material from the self-recoverable layer contains an oxide having the same composition as that of the outermost surface oxide covering layer.

DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and for further advantages thereof, reference is made to the following Detailed Description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a top view showing the concept of a self-recoverable covering material according to an example of the present invention;

FIG. 2 is a life diagnostic circuit diagram showing another example of the present invention;

FIG. 3 is a three-dimensional view showing the construction of a self-recoverable layer by a laminating method according to a further example of the present invention;

FIG. 4 is a view showing the appearance of a ceramic gas turbine moving blade to which the present invention is applied;

FIG. 5 is a view showing the appearance of a ceramic gas turbine stator blade to which the present invention is applied;

FIG. 6 is a schematic view of a continuous hot dip metal plating apparatus to which the present invention is applied;

FIG. 7 is a view showing the appearance of a tile for combustor to which the present invention is applied, and also showing the concept of a life diagnostic function;

FIG. 8 are electron micrographs showing the crystal structures of SiC before and after an oxidizing test;

FIG. 9 are electron micrographs showing the crystal structures of Si₃ N₄ before and after an oxidizing test; and

FIG. 10 is a diagram showing the change of weight in the thermal analysis made for a self-recoverable covering material according to an example of the present invention.

It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

DETAILED DESCRIPTION

Prior to the description of specific embodiments, the feature of a self-recoverable covering material of the present invention will be described below.

One embodiment of a self-recoverable layer according to the present invention includes a boron compound or silicon compound. The compound reacts with an oxidizing atmosphere, and produces a reaction product material capable of recovering cracks. In the present invention, to achieve the self-recovering function, the oxidized product material generated from the self-recoverable layer contains an oxide having the same composition as that of the outermost surface oxide covering layer. Moreover, in order that the self-recoverable layer can fully achieve the self-recovering function, the outermost surface covering layer resisting the environment is made of an oxide.

For example, according to one embodiment of the invention, the surface of a gas turbine moving blade made of silicon nitride is covered with a self-recoverable layer made of ZrB₂ and an outermost surface made of ZrO₂. In such a structure, cracks are given on the surface of the ZrO₂ layer. The structure is then kept in a high temperature oxidizing atmosphere at 1500° C. In this example, the ZrB₂ layer is oxidized to produce compounds of ZrO₂, B₂ O₃ and ZrBO, and the cracks on the outermost ZrO₂ layer are recovered by the compounds containing the ZrO₂ component. The covering layer thus obtained has a reliability higher than that in an conventional structure covered with only the ZrO₂ layer. Here, the oxidizing rate of ZrB₂ is more than that of ZrO₂. ZrO₂ is stable at high temperatures, which has a vapor pressure of 5.6×10⁻⁸ N/m² in the atmospheric air at 1500° C. FIG. 1 is a view showing the concept of the self-recovering function.

According to various embodiments of the present invention, the surface of a member formed of at least one kind of ceramics, metals and C/C composites is covered with an oxide, and a layer containing at least a boron compound or silicon compound between the oxide and the member. In this member, thermal expansion coefficients of the member, intermediate layer, and oxide layer are gradually changed. This is because the thermal impact generates a lot of cracks, due to the difference in the thermal expansion coefficient, not only on the outermost surface oxide but also on the self-recoverable layer. In particular, when the difference in the thermal expansion coefficient is large, not only vertical cracks but also transverse cracks are generated, which causes the peeling of the layers.

To achieve the self-recovering function in some specific embodiments of the present invention, the self-recoverable layer comprises at least one kind of borides of Zr, Hf, Ta, Al and Y or at least one kind of silicates of Zr, Hf, Ta, Al and Y, in an amount of 30 vol % or more. When the content of the borides or silicates is less than 30 vol %, the dispersibility is poor as a whole, and, therefore, the recovering function cannot be fully achieved.

According to some embodiments, the self-recoverable layer includes one or more layers. To enhance the reliability, two or three layers are preferably provided.

According to yet a further embodiment of the invention, reaction product material obtained by the self-recovering function, mainly comprises one kind or more crystalline materials selected from a group consisting of: ZrO₂, Al₂ O₃, HfO₂, zircon (ZrO₂ /SiO₂), mullite (3Al₂ O₃ /2SiO₂), ZrO₂ and B₂ O₃, Al₂ O₃ and B₂ O₃, zircon (ZrO₂ /SiO₂) and B₂ O₃, mullite (3Al₂ O₃ /2SiO₂) and B₂ O₃, HfO₂ and B₂ O₃, Y₂ O₃ and B₂ O₃, Ta₂ O₅ and B₂ O₃, HfO₂ and SiO₂, Y₂ O₃ and SiO₂, and Ta₂ O₅ and SiO₂. Alternately, the reaction product material comprises one kind or more of crystalline materials selected from a group consisting of Al₂ O₃ --BaO, Al₂ O₃ --BeO, Al₂ O₃ --MgO, Al₂ O₃ --MnO, Al₂ O₃ --SiO₂, Al₂ O₃ --TiO₂, Al₂ O₃ --ZnO, Al₂ O₃ --ZrO₂, ZrO₂ --TiO₂, ZrO₂ --SiO₂, ZrO₂ --Nb₂ O₅, K₂ O--Al₂ O₃ --SiO₂, MgO--Al₂ O₃ --SiO₂, Al₂ O₃ --CeO₂, CaO--Al₂ O₃ --SiO₂, BaO--Al₂ O₃ --SiO₂, FeO--Al₂ O₃ --SiO₂, CaO--MgO--SiO₂, CaO--ZrO₂ --SiO₂, MgO--CaO, MgO--SiO₂, Hexacelsian, MgO--TiO₂, MgO--ZrO₂, and SiO₂ --TiO₂. The above oxides such as ZrO₂, Al₂ O₃, MgO, Y₂ O₃ are stable at high temperatures, which have vapor pressures of 1×10⁻⁴ N/m² or less in the atmospheric air at 1500° C.

When the self-recoverable layer is formed of vitreous oxides of B, Ba, Mg, Ti, Li, Na, Nb, K, and V, the temporary recovering is possible; however, the glass having a high vapor pressure disappears by vaporization, and does not act as the recoverable layer. The product material generated from the self-recovering layer must be stable in the service environment.

The present invention is not dependent on the structure to be covered. Therefore, the structure to be covered is not particularly limited, and, according to example embodiments comprise ceramics, metals and/or C/C composites. Examples of ceramic structure to be covered includes: SiC, mullite, Si₃ N₄, sialon, AlN, and Si₂ N₂ O, and composite materials, for example: SiC, ZrO₂, mullite, Si₃ N₄, AlN, Al₂ O₃, B₄ C, BN, Si₂ N₂ O, or Y₂ O₃. According to embodiments of the invention, covered composites have a nanocomposite structure where different particles are dispersed within matrix particles and/or at the grain boundaries. According to other embodiments, composites to be covered include, for example, a reaction-sintered composite material containing 5 to 60 vol % of inorganic compound composed of at least one kind of oxide, carbide, nitride and carbide-nitride and 95 to 40 vol % of metal nitride, which has a structure where the inorganic compound particles are bonded with each other by means of the metal nitride. According to still further embodiments, composites to be covered include metals and alloys such as Fe alloy, Al alloy, Ti alloy, Ni alloy, intermetallic compound, one-way solidified metal monocrystalline metal, cast iron, carbon steel, and heat-resisting steel.

As yet a further example of an embodiment of the present invention, there is provided a life diagnostic circuit on a mechanical structural component having a self-recoverable covering layer. In this case, the life diagnostic circuit is not provided throughout the whole component, but is provided only near the portion where the maximum stress is generated or only at the plane where oxidization is generated. However, the life diagnostic circuit must be provided near the surface of the self-recoverable layer (within 5 mm from the surface). In the structural member, cracks are first generated on the surface and generates the breakage; accordingly, it is required to sensitively detect the generation of the cracks on the surface of the self-recoverable layer. A bundle of fibers (usually: 100 to 1000) are provided as a resistance measuring circuit for life diagnosis. As cracks propagated from the surface cut the fibers, the resistivity is increased. When the resistivity exceeds the set-up threshold value, it is judged that maintenance is required. The equipment is stopped and the parts are exchanged. In the case of a bundle of long fibers, a bundle of combined short fibers or a bundle of fibers of combined particles, cracks cut the fibers in the bundle little by little, so that the resistivity is also increased little by little.

A conductive circuit may be provided near the covering layer of a mechanical structural component having a self-recoverable covering layer. The life of the self-recovering function can be predicted by measuring the change in the resistivity of the conductive circuit (FIG. 2). The conductive circuit for measuring the life is formed of fibers and particles having a low resistivity such as C, SiC, TiN, TiC, ZrN and Cr₂ N. It may be formed of two kinds of fibers or particles having different elasticities for enhancing the efficiency of the life diagnosis. In particular, it is important that the conductive circuit for measuring a life serves as a reinforcing material for the mechanical structural component. The resistivity of the conductive circuit for measuring a life is changed due to the phenomena generated on a bundle of fibers constituting the conductive circuit, such as drawing, cutting, interface (contact) change, deformation, chemical corrosion, and temperature change. The conductive circuit measures the above change in the resistivity, and compares it with a threshold value, thus performing the prediction of a life, diagnosis of the life and the diagnosis of maintenance. For example, when cracks are generated on the surface and the self-recoverable layer is oxidized for achieving the recovering function, the further the oxidization proceeds, the conductive circuit for measuring the life is oxidized, thus increasing the resistance. The value thus obtained is read out, and is compared with a threshold value by a computer provided on the outside of the system. When the value exceeds the threshold value, the stopping signal is automatically outputted. Thus, it becomes possible to stop the equipment before breakage of the mechanical structural component, and to exchange the parts.

The change in the resistance value is supplied to a system, and the system is automatically stopped when the resistance value exceeds a threshold value, thereby performing the maintenance and checking.

Various embodiments of the covering method of the present invention include spraying, chemical deposition, physical deposition, coating, dipping, laminating, and the like. According to some embodiments, the actual covering method is selected based on adhesiveness and does not exert an effect on the self-recovering function. For metals and C/C composites, plasma spraying is desirable. For ceramics, plasma spraying, laminating and chemical deposition are desirable.

According to still a further embodiment of the present invention, a barrier for preventing oxidization is interposed between the surface of the structure made of a ceramic or C/C composite and the self-recoverable layer.

The present invention will be more clearly understood with reference to the following examples:

EXAMPLES 1 TO 95

The surface of each of structures made of ceramics and C/C composites shown in Table 1 was covered with each self-recoverable layer shown in Table 1 to a thickness of 200 μm, and each outermost surface layer such as ZrO₂ shown in Table 1 to a thickness of 200 μm by a powder type plasma spraying. The plasma spraying was performed by a method wherein plasma was generated using argon gas in a vacuum atmosphere. The self-recoverable layer and the outermost surface layer thus obtained were dense with a porosity of 3% or less. A crack having a width of 0.2 mm and a depth of 200 μm was given on the outermost surface of the structure. The structure was then kept for 100 hr in the atmospheric air at 1500° C. As a result, the crack portion was recovered by a product material generated by the reaction between the self-recoverable layer and oxygen in the atmospheric air at 1500 degrees C. Each component of the recovered portion was shown in Tables 1 to 3. The concept of the recovering function is shown in FIG. 1.

As a comparative example, a SiC sintered body was covered with ZrO₂ to a thickness of 200 μm without any self-recoverable layer. A crack having a width of 0.2 mm and a depth of 200 μm was given on the outermost surface of the SiC sintered body. The structure was then kept for 100 hr. in the atmospheric air at 1500° C.

FIG. 8 shows electron micrographs of the above SiC sintered body before and after the oxidizing test. In these micrographs, multiple large voids were generated because SiO₂ was produced on the cracked portion of the surface of the SiC sintered body and then evaporated, as a result of which the SiC sintered body was eroded. As another comparative example, a Si₃ N₄ sintered body was covered with ZrO₂ to a thickness of 200 μm without any self-recoverable layer. A crack having a width of 0.2 mm and a depth of 200 μm was given on the outermost surface of the Si₃ N₄ sintered body. The structure was then kept for 100 hr. in the atmospheric air at 1500° C.

FIG. 9 shows electron micrographs of the above Si₃ N₄ sintered body before and after the oxidizing test. In these micrographs, a SiO₂ oxide film was produced on the cracked portion of the surface of the Si₃ N₄ sintered body and simultaneously an oxide of Yb as a sintering assist agent was melted and precipitated on the surface layer, as a result of which the Si₃ N₄ sintered body was eroded. Consequently, it was revealed that the surface of SiC or Si₃ N₄ sintered body with no self-recoverable layer was eroded by oxidization. The erosion causes the reduction in the reliability.

In these examples, it is important that the oxidizing rate of the self-recoverable layer is more than that of the outermost surface layer. The effect of the self-recoverable layers of the present invention is not limited to the ceramics and C/C composites used in these examples, but may be applied to all of ceramics, C/C composites and metals.

The outermost surface layer may be made of a compound resisting an outer environment, preferably, a compound such as ZrO₂, Al₂ O₃, Y₂ O₃, HfO₂ or Ta₂ O₅ for a high temperature environment. In combination with the above compound, a self-recoverable layer is suitably selected from a boron of Zr, Al, Y, Hf or Ta.

B₄ C, B₄ C+B, or the like used for a self-recoverable layer has no self-recovering function. The reason for this is that CO gas is generated when B₂ O₃ glass is produced by the reaction between B₄ C, or B₄ C+B, and oxygen in the atmospheric air; the reaction gas generates voids or cracks at the recovered portion; and the B₂ O₃ glass is vaporized because of its large vapor pressure.

                  TABLE 1                                                          ______________________________________                                                        Self-                                                                          Recoverable                                                                              Outermost                                             No.   Structure                                                                               Layer     Surface Layer                                                                           Recovered Product                            ______________________________________                                         1     Si.sub.3 N.sub.4                                                                        ZrB.sub.2 ZrO.sub.2                                                                               ZrO.sub.2, B.sub.2 O.sub.3                   2     Si.sub.3 N.sub.4                                                                        TiB.sub.2 TiO.sub.2                                                                               TiO.sub.2, B.sub.2 O.sub.3                   3     Si.sub.3 N.sub.4                                                                        ZrB.sub.2 + HfB.sub.2                                                                    ZrO.sub.2                                                                               ZrO.sub.2, HfO.sub.2                         4     Si.sub.3 N.sub.4                                                                        CaB.sub.2 CaO      CaO, B.sub.2 O.sub.3                         5     Si.sub.3 N.sub.4                                                                        WB        WO.sub.2 WO.sub.2, B.sub.2 O.sub.3                    6     Si.sub.3 N.sub.4                                                                        LaB.sub.2 La.sub.2 O.sub.3                                                                        La.sub.2 O.sub.3, B.sub.2 O.sub.3            7     Si.sub.3 N.sub.4                                                                        HfB.sub.2 HfO.sub.2                                                                               HfO.sub.2, B.sub.2 O.sub.3                   8     Si.sub.3 N.sub.4                                                                        TaB.sub.2 Ta.sub.2 O.sub.5                                                                        Ta.sub.2 O.sub.5, B.sub.2 O.sub.3            9     Si.sub.3 N.sub.4                                                                        NbB.sub.2 Nb.sub.2 O.sub.5                                                                        Nb.sub.2 O.sub.5, B.sub.2 O.sub.3            10    Si.sub.3 N.sub.4                                                                        FeB.sub.2 Fe.sub.2 O.sub.3                                                                        Fe.sub.2 O.sub.3, B.sub.2 O.sub.3            11    Si.sub.3 N.sub.4                                                                        AlB.sub.2 Al.sub.2 O.sub.3                                                                        Al.sub.2 O.sub.3, B.sub.2 O.sub.3            12    Si.sub.3 N.sub.4                                                                        ZrB.sub.2 + SiO.sub.2                                                                    ZrO.sub.2                                                                               ZrO.sub.2, ZrSiO.sub.4                       13    Si.sub.3 N.sub.4                                                                        HfB.sub.2 + SiO.sub.2                                                                    HfO.sub.2                                                                               HfO.sub.2, HfSiO.sub.4                       14    Si.sub.3 N.sub.4                                                                        ZrB.sub.2 + TiB.sub.2                                                                    ZrO.sub.2                                                                               ZrO.sub.2, B.sub.2 O.sub.3, TiO.sub.2        15    Si.sub.3 N.sub.4                                                                        ZrB.sub.2 + FeB.sub.2                                                                    ZrO.sub.2                                                                               Fe.sub.2 O.sub.3, B.sub.2 O.sub.3,                                             ZrO.sub.2                                    16    SiC      ZrB.sub.2 ZrO.sub.2                                                                               ZrO.sub.2, B.sub.2 O.sub.3                   17    SiC      TiB.sub.2 TiO.sub.2                                                                               TiO.sub.2, B.sub.2 O.sub.3                   18    SiC      CaB.sub.2 CaO      CaO, B.sub.2 O.sub.3                         19    SiC      WB        WO.sub.2 WO.sub.2, B.sub.2 O.sub.3                    20    SiC      LaB.sub.2 La.sub.2 O.sub.3                                                                        La.sub.2 O.sub.3, B.sub.2 O.sub.3            21    SiC      HfB.sub.2 HfO.sub.2                                                                               HfO.sub.2, B.sub.2 O.sub.3                   22    SiC      TaB.sub.2 Ta.sub.2 O.sub.5                                                                        Ta.sub.2 O.sub.5, B.sub.2 O.sub.3            23    SiC      NbB.sub.2 Nb.sub.2 O.sub.5                                                                        Nb.sub.2 O.sub.5, B.sub.2 O.sub.3            24    SiC      FeB.sub.2 Fe.sub.2 O.sub.3                                                                        Fe.sub.2 O.sub.3, B.sub.2 O.sub.3            25    SiC      AlB.sub.2 Al.sub.2 O.sub.3                                                                        Al.sub.2 O.sub.3, B.sub.2 O.sub.3            26    SiC      ZrB.sub.2 + SiO.sub.2                                                                    ZrO.sub.2                                                                               ZrO.sub.2, ZrSiO.sub.4                       27    SiC      HfB.sub.2 + SiO.sub.2                                                                    HfO.sub.2                                                                               HfO.sub.2, HfSiO.sub.4                       28    SiC      ZrB.sub.2 + TiB.sub.2                                                                    ZrO.sub.2                                                                               ZrO.sub.2, B.sub.2 O.sub.3, TiO.sub.2        29    SiC      ZrB.sub.2 + FeB.sub.2                                                                    ZrO.sub.2                                                                               Fe.sub.2 O.sub.3, B.sub.2 O.sub.3,ZrO.su                                       b.2                                          30    AlN      ZrB.sub.2 ZrO.sub.2                                                                               ZrO.sub.2,B.sub.2 O.sub.3                    31    AlN      TiB.sub.2 TiO.sub.2                                                                               TiO.sub.2, B.sub.2 O.sub.3                   32    AlN      CaB.sub.2 CaO      CaO, B.sub.2 O.sub.3                         33    AlN      WB        WO.sub.2 WO.sub.2, B.sub.2 O.sub.3                    34    AlN      LaB.sub.2 La.sub.2 O.sub.3                                                                        La.sub.2 O.sub.3, B.sub.2 O.sub.3            35    AlN      HfB.sub.2 HfO.sub.2                                                                               HfO.sub.2, B.sub.2 O.sub.3                   36    AlN      TaB.sub.2 Ta.sub.2 O.sub.5                                                                        Ta.sub.2 O.sub.5, B.sub.2 O.sub.3            37    AlN      NbB.sub.2 Nb.sub.2 O.sub.5                                                                        Nb.sub.2 O.sub.5, B.sub.2 O.sub.3            ______________________________________                                    

                  TABLE 2                                                          ______________________________________                                                        Self-                                                                          Recoverable                                                                              Outermost                                             No.  Structure Layer     Surface Layer                                                                           Recovered Product                            ______________________________________                                         38   AlN       FeB.sub.2 Fe.sub.2 O.sub.3                                                                        Fe.sub.2 O.sub.3, B.sub.2 O.sub.3            39   AlN       AlB.sub.2 Al.sub.2 O.sub.3                                                                        Al.sub.2 O.sub.3, B.sub.2 O.sub.3            40   AlN       ZrB.sub.2 + SiO.sub.2                                                                    ZrO.sub.2                                                                               ZrO.sub.2, ZrSiO.sub.4                       41   AlN       HfB.sub.2 + SiO.sub.2                                                                    HfO.sub.2                                                                               HfO.sub.2, HfSiO.sub.4                       42   AlN       ZrB.sub.2 + TiB.sub.2                                                                    ZrO.sub.2                                                                               ZrO.sub.2, B.sub.2 O.sub.3, TiO.sub.2        43   AlN       ZrB.sub.2 + FeB.sub.2                                                                    ZrO.sub.2                                                                               Fe.sub.2 O.sub.3, B.sub.2 O.sub.3,                                             ZrO.sub.2                                    44   Sialon    ZrB.sub.2 ZrO.sub.2                                                                               ZrO.sub.2, B.sub.2 O.sub.3                   45   Sialon    TiB.sub.2 TiO.sub.2                                                                               TiO.sub.2, B.sub.2 O.sub.3                   46   Sialon    CaB.sub.2 CaO      CaO, B.sub.2 O.sub.3                         47   Sialon    WB        WO.sub.2 WO.sub.2, B.sub.2 O.sub.3                    48   Sialon    LaB.sub.2 La.sub.2 O.sub.3                                                                        La.sub.2 O.sub.3, B.sub.2 O.sub.3            49   Sialon    HfB.sub.2 HfO.sub.2                                                                               HfO.sub.2, B.sub.2 O.sub.3                   50   Sialon    TaB.sub.2 Ta.sub.2 O.sub.5                                                                        Ta.sub.2 O.sub.5, B.sub.2 O.sub.3            51   Sialon    NbB.sub.2 Nb.sub.2 O.sub.5                                                                        Nb.sub.2 O.sub.5, B.sub.2 O.sub.3            52   Sialon    FeB.sub.2 Fe.sub.2 O.sub.3                                                                        Fe.sub.2 O.sub.3, B.sub.2 O.sub.3            53   Sialon    AlB.sub.2 ZrO.sub.2, Al.sub.2 O.sub.3                                                             Al.sub.2 O.sub.3, B.sub.2 O.sub.3            54   Sialon    ZrB.sub.2 + SiO.sub.2                                                                    ZrO.sub.2                                                                               ZrO.sub.2, ZrSiO.sub.4                       55   Sialon    HfB.sub.2 + SiO.sub.2                                                                    HfO.sub.2                                                                               HfO.sub.2, HfSiO.sub.4                       56   Sialon    ZrB.sub.2 + TiB.sub.2                                                                    ZrO.sub.2                                                                               ZrO.sub.2, B.sub.2 O.sub.3, TiO.sub.2        57   Sialon    ZrB.sub.2 + FeB.sub.2                                                                    ZrO.sub.2                                                                               Fe.sub.2 O.sub.3, B.sub.2 O.sub.3,                                             ZrO.sub.2                                    58   Mullite   ZrB.sub.2 ZrO.sub.2                                                                               ZrO.sub.2, B.sub.2 O.sub.3                   59   Mullite   TiB.sub.2 ZrO.sub.2, TiO.sub.2                                                                    TiO.sub.2, B.sub.2 O.sub.3                   60   Mullite   CaB.sub.2 CaO      CaO, B.sub.2 O.sub.3                         61   Mullite   WB        WO.sub.2 WO.sub.2, B.sub.2 O.sub.3                    62   Mullite   LaB.sub.2 La.sub.2 O.sub.3                                                                        La.sub.2 O.sub.3, B.sub.2 O.sub.3            63   Mullite   HfB.sub.2 HfO.sub.2                                                                               HfO.sub.2, B.sub.2 O.sub.3                   64   Mullite   TaB.sub.2 Ta.sub.2 O.sub.5                                                                        TaO.sub.5, B.sub.2 O.sub.3                   65   Mullite   NbB.sub.2 Nb.sub.2 O.sub.5                                                                        Nb.sub.2 O.sub.5, B.sub.2 O.sub.3            66   Mullite   FeB.sub.2 Fe.sub.2 O.sub.3                                                                        Fe.sub.2 O.sub.3, B.sub.2 O.sub.3            67   Mullite   AlB.sub.2 Al.sub.2 O.sub.3                                                                        Al.sub.2 O.sub.3, B.sub.2 O.sub.3            68   Mullite   ZrB.sub.2 + SiO.sub.2                                                                    ZrO.sub.2                                                                               ZrO.sub.2, ZrSiO.sub.4                       69   Mullite   HfB.sub.2 + SiO.sub.2                                                                    HfO.sub.2                                                                               HfO.sub.2, HfSiO.sub.4                       70   Mullite   ZrB.sub.2 + TiB.sub.2                                                                    ZrO.sub.2                                                                               ZrO.sub.2, B.sub.2 O.sub.3, TiO.sub.2        71   Mullite   ZrB.sub.2 + FeB.sub.2                                                                    ZrO.sub.2                                                                               Fe.sub.2 O.sub.3, B.sub.2 O.sub.3,                                             ZrO.sub.2                                    72   Sialon + SiC                                                                             ZrB.sub.2 ZrO.sub.2                                                                               ZrO.sub.2, B.sub.2 O.sub.3                   73   Sialon + TiN                                                                             TiB.sub.2 ZrO.sub.2, TiO.sub.2                                                                    TiO.sub.2, B.sub.2 O.sub.3                   ______________________________________                                    

                  TABLE 3                                                          ______________________________________                                                          Self-     Outermost                                                            Recoverable                                                                              Surface                                             No.   Structure  Layer     Layer  Recovered Product                            ______________________________________                                         74    Sialon + ZrO.sub.2                                                                        CaB.sub.2 CaO    CaO, B.sub.2 O.sub.3                         75    Si.sub.3 N.sub.4 + SiC                                                                    WB        WO.sub.2                                                                              WO.sub.2, B.sub.2 O.sub.3                    76    Si.sub.3 N.sub.4 + AlN                                                                    LaB.sub.2 La.sub.2 O.sub.3                                                                      La.sub.2 O.sub.3, B.sub.2 O.sub.3            77    Si.sub.3 N.sub.4 + ZrO.sub.2                                                              HfB.sub.2 HfO.sub.2                                                                             HfO.sub.2, B.sub.2 O.sub.3                   78    Si.sub.2 N.sub.2 O                                                                        TaB.sub.2 Ta.sub.2 O.sub.5                                                                      Ta.sub.2 O.sub.5, B.sub.2 O.sub.3            79    Si.sub.3 N.sub.4 + TiCN                                                                   NbB.sub.2 Nb.sub.2 O.sub.5                                                                      Nb.sub.2 O.sub.5, B.sub.2 O.sub.3            80    Si.sub.3 N.sub.4 + TiN                                                                    FeB.sub.2 Fe.sub.2 O.sub.3                                                                      Fe.sub.2 O.sub.3, B.sub.2 O.sub.3            81    Si.sub.3 N.sub.4 + Mo                                                                     AlB.sub.2 Al.sub.2 O.sub.3                                                                      Al.sub.2 O.sub.3, B.sub.2 O.sub.3            82    Sialon + Mo                                                                               ZrB.sub.2 + TiB.sub.2                                                                    ZrO.sub.2                                                                             ZrO.sub.2, B.sub.2 O.sub.3, TiO.sub.2        83    TiN        ZrB.sub.2 + FeB.sub.2                                                                    ZrO.sub.2                                                                             Fe.sub.2 O.sub.3, B.sub.2 O.sub.3,                                             ZrO.sub.2                                    84    BN         AlB.sub.2 Al.sub.2 O.sub.3                                                                      Al.sub.2 O.sub.3, B.sub.2 O.sub.3            85    C/C composite                                                                             ZrB.sub.2 ZrO.sub.2                                                                             ZrO.sub.2, B.sub.2 O.sub.3                   86    C/C composite                                                                             TiB.sub.2 TiO.sub.2                                                                             TiO.sub.2, B.sub.2 O.sub.3                   87    C/C composite                                                                             CaB.sub.2 CaO    CaO, B.sub.2 O.sub.3                         88    C/C composite                                                                             WB        WO.sub.2                                                                              WO.sub.2, B.sub.2 O.sub.3                    89    C/C composite                                                                             LaB.sub.2 La.sub.2 O.sub.3                                                                      La.sub.2 O.sub.3, B.sub.2 O.sub.3            90    C/C composite                                                                             HfB.sub.2 HfO.sub.2                                                                             HfO.sub.2, B.sub.2 O.sub.3                   91    C/C composite                                                                             TaB.sub.2 Ta.sub.2 O.sub.5                                                                      Ta.sub.2 O.sub.5, B.sub.2 O.sub.3            92    C/C composite                                                                             NbB.sub.2 Nb.sub.2 O.sub.5                                                                      Nb.sub.2 O.sub.5, B.sub.2 O.sub.3            93    C/C composite                                                                             ZrB.sub.2 ZrO.sub.2                                                                             ZrO.sub.2, B.sub.2 O.sub.3                   94    C/C composite                                                                             ZrB.sub.2 + TiB.sub.2                                                                    ZrO.sub.2                                                                             ZrO.sub.2, B.sub.2 O.sub.3, TiO.sub.2        95    C/C composite                                                                             ZrB.sub.2 + FeB.sub.2                                                                    ZrO.sub.2                                                                             Fe.sub.2 O.sub.3, B.sub.2 O.sub.3,                                             ZrO.sub.2                                    ______________________________________                                    

EXAMPLES 96-102

Tests were made to examine the suitability of self-recoverable layers. The surface of a sample (10×10×5 mm) made of each structure shown in Table 4 was covered with each self-recoverable layer shown in Table 4 to a thickness of 200 μm, and each outermost surface layer such as ZrO₂ shown in Table 4 to a thickness of 200 μm by a powder type plasma spraying. A crack having a width of 0.2 mm and a depth of 200 μm was given on the surface of the sample. The sample was then kept for 100 hr. in the atmospheric air at 1500° C., which gave the result shown in Table 4. It was revealed that single B₄ C, single B and (B₄ C+SiO₂) are evaporated and they had no recovering function for cracks.

Next, the change of a self-recoverable layer by the high temperature oxidization was measured by a thermobalance, which gave the result shown in FIG. 10. From this figure, it was confirmed that single B₄ C, single B were perfectly evaporated by oxidization and (B₄ C+SiO₂) was reduced in weight by 50 wt % by oxidization.

                  TABLE 4                                                          ______________________________________                                                       Self-     Outermost                                                            Recoverable                                                                              Surface                                                No.  Structure                                                                               Layer     Layer  Recovered Product                               ______________________________________                                         96   Si.sub.3 N.sub.4                                                                        B.sub.4 C ZrO.sub.2                                                                             Absence of recovering                                                          function Si.sub.3 N.sub.4, eroded               97   SiC      BN        ZrO.sub.2                                                                             Absence of recovering                                                          function SiC, eroded                            98   SiC      B         ZrO.sub.2                                                                             Absence of recovering                                                          function SiC, eroded                            99   C/C      B.sub.4 C + SiO.sub.2                                                                    SiC    Absence of recovering                                                          function C/C, eroded                            100  C/C      BN + SiO.sub.2                                                                           SiC    Absence of recovering                                                          function C/C, eroded                            101  C/C      B + SiO.sub.2                                                                            SiC    Absence of recovering                                                          function C/C, eroded                            102  C/C      ZrB.sub.2 ZrO.sub.2                                                                             Presence of recovering                                                         function C/C, recovered by                                                     ZrO.sub.2 and the like                          ______________________________________                                    

EXAMPLES 103-132

The surface of each of the structures made of ceramics and C/C composites shown in Table 5 was covered with each self-recoverable layer shown in Table 5 to a thickness of 200 μm, and an outermost surface layer made of each oxide shown in Table 5 to a thickness of 150 μm by a powder type plasma spraying. The plasma spraying was performed by a method wherein plasma was generated using argon gas in a vacuum atmosphere. The self-recoverable layer and the outermost layer thus obtained were dense with a porosity of 3% or less. A crack having a width of 0.2 mm and a depth of 150 μm was given on the outermost surface of the structure. The structure was then kept for 100 hr. in the atmospheric air at 1500° C. As a result, the cracked portion was recovered by a product material generated by the reaction between the self-recoverable layer and oxygen in the atmospheric air. Each component of the recovered portion was shown in Table 5.

                  TABLE 5                                                          ______________________________________                                                        Self-                                                                          Recoverable                                                                              Outermost                                             No.  Structure Layer     Surface Layer                                                                           Recovered Product                            ______________________________________                                         103  Si.sub.3 N.sub.4                                                                         ZrB.sub.2 ZrO.sub.2, Al.sub.2 O.sub.3                                                             ZrO.sub.2, B.sub.2 O.sub.3                   104  Si.sub.3 N.sub.4                                                                         TiB.sub.2 TiO.sub.2, Al.sub.2 O.sub.3                                                             TiO.sub.2, B.sub.2 O.sub.3                   105  SiC       CaB.sub.2 CaO, Al.sub.2 O.sub.3                                                                   CaO, B.sub.2 O.sub.3                         106  Mullite   WB        WO.sub.2, Al.sub.2 O.sub.3                                                              WO.sub.2, B.sub.2 O.sub.3                    107  Sialon    LaB.sub.2 La.sub.2 O.sub.3, Al.sub.2 O.sub.3                                                      La.sub.2 O.sub.3, B.sub.2 O.sub.3            108  C/C       HfB.sub.2 HfO.sub.2, Al.sub.2 O.sub.3                                                             HfO.sub.2, B.sub.2 O.sub.3                        composite                                                                 109  Si.sub.3 N.sub.4 + SiC                                                                   TaB.sub.2 Ta.sub.2 O.sub.5, Al.sub.2 O.sub.3                                                      Ta.sub.2 O.sub.5, B.sub.2 O.sub.3            110  Si.sub.3 N.sub.4                                                                         NbB.sub.2 Nb.sub.2 O.sub.5, Al.sub.2 O.sub.3                                                      Nb.sub.2 O.sub.5, B.sub.2 O.sub.3            111  Si.sub.3 N.sub.4                                                                         FeB.sub.2 Fe.sub.2 O.sub.3, Al.sub.2 O.sub.3                                                      Fe.sub.2 O.sub.3, B.sub.2 O.sub.3            112  Si.sub.3 N.sub.4                                                                         AlB.sub.2 Al.sub.2 O.sub.3                                                                        Al.sub.2 O.sub.3, B.sub.2 O.sub.3            113  Si.sub.3 N.sub.4                                                                         ZrB.sub.2 +                                                                              ZrO.sub.2, Al.sub.2 O.sub.3                                                             ZrO.sub.2, HfO.sub.2, Al.sub.2 O.sub.3                      HfB.sub.2 + AlB.sub.2                                           114  Si.sub.3 N.sub.4 +                                                                       ZrB.sub.2 + TlB.sub.2                                                                    ZrO.sub.2, Al.sub.2 O.sub.3                                                             ZrO.sub.2, B.sub.2 O.sub.3, TiO.sub.3             ZrO.sub.2                                                                 115  Si.sub.3 N.sub.4 + AlN                                                                   ZrB.sub.2 + FeB.sub.2                                                                    ZrO.sub.2, Al.sub.2 O.sub.3                                                             Fe.sub.2 O.sub.3, B.sub.2 O.sub.3,                                             ZrO.sub.2                                    116  SiC       ZrB.sub.2 Zircon   ZrO.sub.2, B.sub.2 O.sub.3                   117  SiC       TlB.sub.2 TiO.sub.2, Cr.sub.2 O.sub.3                                                             ZrO.sub.2, B.sub.2 O.sub.3                   118  Si.sub.3 N.sub.4 + SiC                                                                   CaB.sub.2 CaO, Cr.sub.2 O.sub.3                                                                   CaO, B.sub.2 O.sub.3                         119  AlN       WB        WO.sub.2, TiO.sub.2                                                                     WO.sub.2, B.sub.2 O.sub.3                    120  Mullite   LaB.sub.2 La.sub.2 O.sub.3, SiO.sub.2                                                             La.sub.2 O.sub.3, B.sub.2 O.sub.3            121  Sialon    YB.sub.2  Y.sub.2 O.sub.3                                                                         Y.sub.2 O.sub.3, B.sub.2 O.sub.3             122  C/C       YB.sub.2  ZrO.sub.2, Y.sub.2 O.sub.3                                                              Y.sub.2 O.sub.3, B.sub.2 O.sub.3                  composite                                                                 123  SiC       NbB.sub.2 Y.sub.2 O.sub.3                                                                         Nb.sub.2 O.sub.5, B.sub.2 O.sub.3            124  Si.sub.2 N.sub.2 O                                                                       ZrB.sub.2 Zircon   ZrO.sub.2, B.sub.2 O.sub.3                   125  Sialon +  AlB.sub.2 Al.sub.2 O.sub.3                                                                        Al.sub.2 O.sub.3, B.sub.2 O.sub.3                 ZrO.sub.2                                                                 126  Sialon + TiN                                                                             ZrB.sub.2 + TiB.sub.2                                                                    ZrO.sub.2, Y.sub.2 O.sub.3                                                              ZrO.sub.2, B.sub.2 O.sub.3, TiO.sub.2        127  Sialon + SiC                                                                             ZrB.sub.2 + FeB.sub.2                                                                    ZrO.sub.2, Cr.sub.2 O.sub.3                                                             Fe.sub.2 O.sub.3, B.sub.2 O.sub.3,                                             ZrO.sub.2                                    128  Si.sub.3 N.sub.4 + TiN                                                                   FeB.sub.2 ZrO.sub.2, Cr.sub.2 O.sub.3                                                             ZrO.sub.2, B.sub.2 O.sub.3                   129  Si.sub.3 N.sub.4 + Mo                                                                    AlB.sub.2 Al.sub.2 O.sub.3                                                                        Al.sub.2 O.sub.3, B.sub.2 O.sub.3            130  Sialon + Mo                                                                              ZrB.sub.2 + TiB.sub.2                                                                    Zircon   ZrO.sub.2, B.sub.2 O.sub.3, TiO.sub.2        131  TiN       ZrB.sub.2 + FeB.sub.2                                                                    ZrO.sub.2, Y.sub.2 O.sub.3                                                              Fe.sub.2 O.sub.3, B.sub.2 O.sub.3,                                             ZrO.sub.2                                    132  BN        AlB.sub.2 Al.sub.2 O.sub.3, TiO.sub.2                                                             Al.sub.2 O.sub.3, B.sub.2 O.sub.3            ______________________________________                                    

In these examples, it is important that the oxidizing rate of the self-recoverable layer is greater than that of the outermost surface layer. The effect of the self-recoverable layers of the present invention is not limited to the ceramics and C/C composites used in these examples, but may be applied to all of ceramics, C/C composites and metals.

In comparative examples, shown in Table 6, the oxidizing rate of the self-recoverable layer is less than that of the outermost surface layer. In such a covering structure, cracks generated on the outermost surface layer are temporarily recovered by the oxidizing reaction of the outermost surface layer; however, the essential recovering function cannot be obtained because the whole of the outermost surface layer is oxidized and eroded.

                  TABLE 6                                                          ______________________________________                                         Comparative      Self-Recoverable                                                                           Outermost                                                                               Recovered                                Example No.                                                                            Structure                                                                               Layer       Surface Layer                                                                           Product                                  ______________________________________                                         1       Si.sub.3 N.sub.4                                                                        Al.sub.2 O.sub.3                                                                           ZrB.sub.2                                                                               None                                     2       Si.sub.3 N.sub.4                                                                        Al.sub.2 O.sub.3                                                                           TiB.sub.2                                                                               None                                     3       Si.sub.3 N.sub.4                                                                        SiC         BN       None                                     4       SiC      ZrO.sub.2   ZrB.sub.2                                                                               None                                     ______________________________________                                    

EXAMPLES 133-155

The surface of each of the structures made of metals and alloys shown in Table 7 was covered with each self-recoverable layer shown in Table 7 to a thickness of 200 μm, and an outermost surface layer made of each oxide shown in Table 7 to a thickness of 150 μm by a powder type plasma spraying. The plasma spraying was performed by a method wherein plasma was generated using argon gas in a vacuum atmosphere. The self-recoverable layer and the outermost layer thus obtained were dense with a porosity of 3% or less. A crack having a width of 0.2 mm and a depth of 150 μm was given on the outermost surface of the structure. The structure was then kept for 100 hr. in the atmospheric air at 1500° C., while the interior of the structure was cooled by water. As a result, the cracked portion was recovered by a product material generated by the reaction between the self-recoverable layer and oxygen in the atmospheric air. Each component of the recovered portion was shown in Table 7.

From these examples, it becomes apparent that the self-recovering function of the present invention is not affected by a covering material. Structures to be covered may include ceramics, C/C composites, and metals and alloys.

                                      TABLE 7                                      __________________________________________________________________________                   Self-                                                                          Recoverable                                                                           Outermost                                                 No. Structure Layer  Surface Layer                                                                          Recovered Product                                 __________________________________________________________________________     133 Stainless Steel                                                                          ZrB.sub.2                                                                             ZrO.sub.2                                                                              ZrO.sub.2,B.sub.2 O.sub.3                         134 Stainless Steel                                                                          TiB.sub.2                                                                             TiO.sub.2, Al.sub.2 O.sub.3                                                            TiO.sub.2, B.sub.2 O.sub.3                        135 Stainless Steel                                                                          CaB.sub.2                                                                             CaO, ZrO.sub.2                                                                         CaO, B.sub.2 O.sub.3                              136 Cast iron WB     WO.sub.2, Al.sub.2 O.sub.3                                                             WO.sub.2, B.sub.2 O.sub.3                         137 Nb.sub.3 Al                                                                              LaB.sub.2                                                                             La.sub.2 O.sub.3, Al.sub.2 O.sub.3                                                     La.sub.2 O.sub.3, B.sub.2 O.sub.3                 138 Ni alloy  HfB.sub.2                                                                             HfO.sub.2, Al.sub.2 O.sub.3                                                            HfO.sub.2, B.sub.2 O.sub.3                        139 Ni alloy  TaB.sub.2                                                                             Ta.sub.2 O.sub.5, Al.sub.2 O.sub.3                                                     Ta.sub.2 O.sub.5, B.sub.2 O.sub.3                 140 Cr alloy  NbB.sub.2                                                                             Nb.sub.2 O.sub.5, Al.sub.2 O.sub.3                                                     Nb.sub.2 O.sub.5, B.sub.2 O.sub.3                 141 Carbon steel                                                                             AlB.sub.2                                                                             Al.sub.2 O.sub.3                                                                       Al.sub.2 O.sub.3, B.sub.2 O.sub.3                 142 Cr--Ni alloy                                                                             ZrB.sub.2                                                                             ZrO.sub.2                                                                              ZrO.sub.2, B.sub.2 O.sub.3                        143 Cr--Ni alloy                                                                             ZrB.sub.2  + AlB.sub.2                                                                Al.sub.2 O.sub.3                                                                       ZrO.sub.2, Al.sub.2 O.sub.3                       144 Fee Cutting steel;                                                                       ZrB.sub.2  + FeB.sub.2                                                                ZrO.sub.2, Al.sub.2 O.sub.3                                                            FeO.sub.3, B.sub.2 O.sub.3, ZrO.sub.2             145 Heat-resisting steel                                                                     ZrB.sub.2  + TiB.sub.2                                                                ZrO.sub.2, Al.sub.2 O.sub.3                                                            ZrO.sub.2, B.sub.2 O.sub.3, TiO.sub.2             146 Rolled steel                                                                             NbB.sub.2                                                                             Nb.sub.2 O.sub.5, Al.sub.2 O.sub.3                                                     Nb.sub.2 O.sub.5, B.sub.2 O.sub.3                 147 Inconel   NbB.sub.2                                                                             Nb.sub.2 O.sub.5, ZrO.sub.2                                                            Nb.sub.2 O.sub.5, B.sub.2 O.sub.3                 148 Ni.sub.3 (Al, Ti)                                                                        ZrB.sub.2  + TiB.sub.2                                                                ZrO.sub.2, Al.sub.2 O.sub.3                                                            ZrO.sub.2, B.sub.2 O.sub.3, TiO.sub.2             149 Fiber-reinforced Al                                                                      AlB.sub.2                                                                             Al.sub.2 O.sub.3                                                                       Al.sub.2 O.sub.3, B.sub.2 O.sub.3                 150 SiC particle                                                                             ZrB.sub.2                                                                             ZrO.sub.2                                                                              ZrO.sub.2, B.sub.2 O.sub.3                            dispersion-                                                                    strengthened Ti                                                                metal                                                                      151 One-way eutectic                                                                         TiB.sub.2                                                                             TiO.sub.2, Al.sub.2 O.sub.3                                                            TiO.sub.2, B.sub.2 O.sub.3                            alloy (Cr--TaC)                                                            152 Al alloy  ZrB.sub.2                                                                             ZrO.sub.2                                                                              ZrO.sub.2, B.sub.2 O.sub.3                        153 Ti alloy  ZrB.sub.2                                                                             ZrO.sub.2                                                                              ZrO.sub.2, B.sub.2 O.sub.3                        154 Al alloy  ZrB.sub.2                                                                             ZrO.sub.2, Al.sub.2 O.sub.3                                                            ZrO.sub.2, B.sub.2 O.sub.3                        155 SiC particle                                                                   dispersion-                                                                              ZrB.sub.2                                                                             ZrO.sub.2, SiC                                                                         ZrO.sub.2, B.sub.2 O.sub.3                            strengthened Ti                                                                metal                                                                      __________________________________________________________________________

EXAMPLES 156-164

The surface of each structure shown in Table 8 was covered with three or more layers including a self-recoverable layer having a thickness of 200 μm, and an outermost surface layer having a thickness of 100 μm by a powder type plasma spraying. The plasma spraying was performed by a method wherein plasma was generated using argon gas in a vacuum atmosphere. The self-recoverable layer and the outermost layer thus obtained were dense, with a porosity of 3% or less.

Each structure of the covering layers is shown in Table 8. Each layer such as the self-recoverable layer is dense, with a porosity of 3% or less. A crack having a width of 0.2 mm and a depth of 100 μm up to the self-recoverable layer was given on the outermost surface of the structure. The structure was then kept for 100 hr. in the atmospheric air at 1500° C. As a result, the cracked portion was recovered by a product material generated by the reaction between the self-recoverable layer and oxygen in the atmospheric air at 1500 degrees C.

                                      TABLE 8                                      __________________________________________________________________________               Self-Recoverable Layer and Outermost                                                                 Recovered                                      No  Structure                                                                            Surface Layer         Product                                        __________________________________________________________________________     156 Si.sub.3 N.sub.4                                                                     First layer:                                                                           ZrO.sub.2     ZrO.sub.2, B.sub.2 O.sub.3                               Second layer:                                                                          ZrB.sub.2 self-recoverable                                                     layer                                                                  Third layer:                                                                           ZrO.sub.2                                                    157 Sialon                                                                               First layer:                                                                           Al.sub.2 O.sub.3, TiO.sub.2                                                                  ZrO.sub.2, B.sub.2 O.sub.3                               Second layer:                                                                          ZrB.sub.2 self-recoverable                                                     layer                                                                  Third layer:                                                                           ZrO.sub.2                                                    158 Sialon                                                                               First layer:                                                                           Al.sub.2 O.sub.3, TiO.sub.2                                                                  ZrO.sub.2, B.sub.2 O.sub.3                               Second layer:                                                                          Al.sub.2 O.sub.3                                                       Third layer:                                                                           ZrB.sub.2 self-recoverable                                                     layer                                                                  Fourth layer:                                                                          ZrO.sub.2                                                    159 Inconel                                                                              First layer:                                                                           Al.sub.2 O.sub.3, TiO.sub.2                                                                  Al.sub.2 O.sub.3, B.sub.2 O.sub.3                        Second layer:                                                                          AlB.sub.2 self-recoverable                                                                   ZrO.sub.2, B.sub.2 O.sub.3                                       layer                                                                  Third layer:                                                                           Al.sub.2 O.sub.3                                                       Fourth layer:                                                                          ZrB.sub.2 self-recoverable layer                                       Fifth layer:                                                                           ZrO.sub.2                                                    160 SiC   First layer:.                                                                          Si.sub.3 N.sub.4                                                                             TiO.sub.2, B.sub.2 O.sub.3                               Second layer:                                                                          TiB.sub.2 self-recoverable                                                                   ZrO.sub.2, B.sub.2 O.sub.3                                       layer                                                                  Third layer:                                                                           ZrB.sub.2 self-recoverable                                                     layer                                                                  Fourth layer:                                                                          ZrO.sub.2                                                    161 Sialon                                                                               First layer:                                                                           AlN                                                                    Second layer:                                                                          ZrB.sub.2 self-recoverable                                                                   ZrO.sub.2, B.sub.2 O.sub.3                                       layer                                                                  Third layer:                                                                           ZrO.sub.2, Al.sub.2 O.sub.3                                  162 Si.sub.3 N.sub.4                                                                     First layer:                                                                           Ni alloy      Nb.sub.2 O.sub.3, B.sub.2 O.sub.3                        Second layer:                                                                          NbB.sub.2 self-recoverable                                                                   ZrO.sub.2, B.sub.2 O.sub.3                                       layer                                                                  Third layer:                                                                           ZrB.sub.2 self-recoverable                                                     layer                                                                  Fourth layer:                                                                          ZrO.sub.2                                                    163 Inconel                                                                              First layer:                                                                           Al.sub.2 O.sub.3, TiO.sub.2                                                                  ZrO.sub.2, B.sub.2 O.sub.3                               Second layer:                                                                          TiC                                                                    Third layer:                                                                           Al.sub.2 O.sub.3                                                       Fourth layer:                                                                          ZrB.sub.2 self-recoverable layer                                       Fifth layer:                                                                           ZrO.sub.2                                                    164 Nb.sub.3 Al                                                                          First layer:                                                                           Al.sub.2 O.sub.3                                                                             ZrO.sub.2, B.sub.2 O.sub.3                               Second layer:                                                                          ZrB.sub.2 self-recoverable                                                     layer                                                                  Third layer:                                                                           ZrO.sub.2                                                    __________________________________________________________________________

EXAMPLES 165-173

Tests were made to examine what effect the difference in the thermal expansion coefficient between covering layers would exert on the self-recovering function.

The surface of each structure shown in Table 9 was covered with three or more layers including a self-recoverable layer having a thickness of 200 μm, and an outermost surface layer having a thickness of 100 μm by a powder type plasma spraying. The plasma spraying was performed by a method wherein plasma was generated using argon gas in a vacuum atmosphere. The self-recoverable layer and the outermost surface layer were dense, with a porosity of 3% or less. The structure of the covering layers is shown in Table 9. The structure thus covered was heated from room temperature to 500° C. and then cooled by water. Thus, the structure was examined for the state of the interface between the covering layers.

As a result, it is revealed that when the difference in the thermal expansion coefficient between some covering layers including the self-recoverable layer is increased to be more than 4×10⁻⁶ /°C., peeling is generated, which fails to achieve the self-recovering function with a high reliability. Accordingly, it is possible to obtain covering layers having a self-recovering function being excellent in the thermal shock resistance by controlling the thermal expansion coefficients between respective covering layers to be gradually changed.

                                      TABLE 9                                      __________________________________________________________________________                 Self-Recoverable Layer                                                         and Outermost Surface                                                                      Recovered                                                                             Result of                                       No. Structure                                                                              Layer       Product                                                                               Test                                            __________________________________________________________________________     165 Si.sub.3 N.sub.4                                                                       Second layer:                                                                              TiO.sub.2, B.sub.2 O.sub.3                                                            Partial                                             3 × 10.sup.-6 /°C.                                                        TiB.sub.2 (7.9 × 10.sup.-6 /°C.)                                                     generation                                                  Third layer:       of separation                                               ZrO.sub.2 (8.0 × 10.sup.-6 /°C.)                      166 Si.sub.3 N.sub.4                                                                       Second layer:                                                                              ZrO.sub.2, B.sub.2 O.sub.3                                                            Good                                                4 × 10.sup.-6 /°C.                                                        ZrB.sub.2 (5.8 × 10.sup.-6 /°C.)                                  Third layer:                                                                   ZrO.sub.2 (8.0 × 10.sup.-6 /°C.)                      167 Sialon  Second layer                                                                               ZrO.sub.2, B.sub.2 O.sub.3                                                            Good                                                3.2 × 1O.sup.-6 /°C.                                                      ZrB.sub.2 (5.8 × 10.sup.-6 /°C.)                                  Third layer:                                                                   ZrO.sub.2 (8.0 × 10.sup.-6 /°C.)                      168 Si.sub.3 N.sub.4                                                                       Second layer:                                                                              TiO.sub.2, B.sub.2 O.sub.3                                                            Partial                                             3 × 10.sup.-6 /°C.                                                        TiB.sub.2 (7.9 × 10.sup.-6 /°C.)                                                     generation                                                  Third layer:       of separation                                               ZrO.sub.2, TiO.sub.2 (8.0 × 10.sup.-6 /°C.)           169 Si.sub.3 N.sub.4                                                                       First layer:                                                                               ZrO.sub.2, B.sub.2 O.sub.3                                                            Good                                                3 × 10.sup.-6 /°C.                                                        ZrB.sub.2 + Si.sub.3 N.sub.4                                                   (4.0 × 10.sup.-6 /°C.)                                            Second layer:                                                                  ZrO.sub.2 (8.0 × 10.sup.-6 /°C.)                      170 Carbon steel                                                                           First layer:                                                                               TiO.sub.2, B.sub.2 O.sub.3                                                            Good                                                12 × 10.sup.-6 /°C.                                                       TiO.sub.2 (9.0 × 10.sup.-6 /°C.)                                  Second layer:                                                                  TiB.sub.2 (7.9 × 10.sup.-6 /°C.)                                  Third layer:                                                                   TiO.sub.2, ZrO.sub.2                                                           (8.5 × 10.sup.-6 /°C.)                                171 Carbon steel                                                                           First layer:                                                                               TiO.sub.2, B.sub.2 O.sub.3                                                            Partial                                             12 × 10.sup.-6 /°C.                                                       TiB.sub.2 (7.9 × 10.sup.-6 /°C.)                                                     generation                                                  Second layer:      of separation                                               TiO.sub.2, ZrO.sub.2                                                           (8.5 × 10.sup.-6 /°C.)                                172 Stainless                                                                              First layer:                                                                               ZrO.sub.2, B.sub.2 O.sub.3                                                            Good                                                steel   TiO.sub.2 (9.0 × 10.sup.-6 /°C.)                          13 × 10.sup.-6 /°C.                                                       Second layer:                                                                  ZrO.sub.2 (5.8 × 10.sup.-6 /°C.)                                  Third layer:                                                                   ZrO.sub.2 (8.0 × 10.sup.-6 /°C.)                      173 Stainless                                                                              First layer:                                                                               ZrO.sub.2, B.sub.2 O.sub.3                                                            Partial                                             steel   TiB.sub.2 (7.9 × 10.sup.-6 /°C.)                                                     generation                                          13 × 10.sup.-6 /°C.                                                       Second layer       of separation                                               ZrB.sub.2 (5.8 × 10.sup.-6 /°C.)                                  Third layer:                                                                   ZrO.sub.2 (8.0 × 10.sup.-6 /°C.)                      __________________________________________________________________________

EXAMPLES 174-228

The surface of each structure shown in Table 10 was covered with each self-recoverable layer shown in Table 10 to a thickness of 200 μm, and each outermost surface layer such as ZrO₂ shown in Table 10 to a thickness of 200 μm by a powder type plasma spraying. The plasma spraying was performed by a method wherein plasma was generated using argon gas in a vacuum atmosphere. The self-recoverable layer and the outermost layer thus obtained were dense with a porosity of 3% or less. A crack having a width of 0.2 mm and a depth of 200 μm was given on the outermost surface of the structure. The structure was then kept for 100 hr. in the atmospheric air at 1500° C. As a result, the cracked portion was recovered by a product material generated by the reaction between the self-recoverable layer and oxygen in the atmospheric air at 1500° C. Each component of the recovered portion was shown in Tables 10 and 11.

                                      TABLE 10                                     __________________________________________________________________________                 Self-                                                                          Recoverable                                                                            Outermost                                                  No. Structure                                                                              Layer   Surface Layer                                                                         Recovered Product                                   __________________________________________________________________________     174 Si.sub.3 N.sub.4                                                                       ZrSi    ZrO.sub.2                                                                             ZrO.sub.2, SiO.sub.2                                175 Si.sub.3 N.sub.4                                                                       TiSi    TiO.sub.2, ZrO.sub.2                                                                  TiO.sub.2, SiO.sub.2                                176 Si.sub.3 N.sub.4                                                                       MoSi    ZrO.sub.2, SiO.sub.2                                                                  MoO, SiO.sub.2                                      177 Si.sub.3 N.sub.4                                                                       FeSi    ZrO.sub.2, SiO.sub.2                                                                  Fe.sub.2 O.sub.3, SiO.sub.2                         178 Si.sub.3 N.sub.4                                                                       CoSi    ZrO.sub.2, SiO.sub.2                                                                  CoO.sub.2, SiO.sub.2                                179 Si.sub.3 N.sub.4                                                                       LaSi    ZrO.sub.2, SiO.sub.2                                                                  La.sub.2 O.sub.3, SiO.sub.2                         180 Si.sub.3 N.sub.4                                                                       HfSi    ZrO.sub.2, SiO.sub.2                                                                  HfO.sub.2, SiO.sub.2                                181 Si.sub.3 N.sub.4                                                                       TaSi    ZrO.sub.2, SiO.sub.2                                                                  TaO.sub.2, SiO.sub.2                                182 Si.sub.3 N.sub.4                                                                       NbSi    ZrO.sub.2, SiO.sub.2                                                                  Nb.sub.2 O.sub.5, SiO.sub.2                         183 Si.sub.3 N.sub.4                                                                       AlSi    ZrO.sub.2, Al.sub.2 O.sub.3                                                           Al.sub.2 O.sub.3, SiO.sub.2                         184 Si.sub.3 N.sub.4                                                                       NiSi    ZrO.sub.2, SiO.sub.2                                                                  NiO.sub.2, SiO.sub.2                                185 Si.sub.3 N.sub.4                                                                       ZrB.sub.2  + ZrSi                                                                      ZrO.sub.2, SiO.sub.2                                                                  ZrO.sub.2, B.sub.2 O.sub.3, SiO.sub.2               186 Si.sub.3 N.sub.4                                                                       ZrSi + MoSi                                                                            ZrO.sub.2, SiO.sub.2                                                                  ZrO.sub.2, SiO.sub.2, MoO                           187 Si.sub.3 N.sub.4                                                                       ZrSi + AlSi                                                                            ZrO.sub.2, SiO.sub.2                                                                  ZrO.sub.2, SiO.sub.2, Al.sub.2 O.sub.3              188 SiC     AlSi    Al.sub.2 O.sub.3                                                                      Al.sub.2 O.sub.3, SiO.sub.2                         189 Mullite NiSi    ZrO.sub.2, SiO.sub.2                                                                  NiO.sub.2, SiO.sub.2                                190 Sialon  ZrB.sub.2  + Zrsi                                                                      ZrO.sub.2, SiO.sub.2                                                                  ZrO.sub.2, B.sub.2 O.sub.3, SiO.sub.2               191 AlN     ZrSi + MoSi                                                                            ZrO.sub.2, Al.sub.2 O.sub.3                                                           ZrO.sub.2, SiO.sub.2, MoO                           192 C/C composite                                                                          ZrB.sub.2 + ZrSi                                                                       ZrO.sub.2, SiO.sub.2                                                                  ZrO.sub.2, B.sub.2 O.sub.3, SiO.sub.2               193 Sialon + SiC                                                                           ZrB.sub.2  + ZrSi                                                                      ZrO.sub.2                                                                             ZrO.sub.2, B.sub.2 O.sub.3, SiO.sub.2               194 Sialon + TiN                                                                           ZrSi + B.sub.4 C                                                                       ZrO.sub.2                                                                             ZrO.sub.2, SiO.sub.2, B.sub.2 O.sub.3, SiC          195 C/C composite                                                                          AlSi    Al.sub.2 O.sub.3                                                                      Al.sub.2 O.sub.3, SiO.sub.2                         196 Sialon + ZrO.sub.2                                                                     ZrSi + MoSi                                                                            ZrO.sub.2                                                                             ZrO.sub.2, SiO.sub.2, MoO                           197 Si.sub.3 N.sub.4 + SiC                                                                 ZrSi + MoSi                                                                            ZrO.sub.2                                                                             ZrO.sub.2, SiO.sub.2, MoO                           198 Si.sub.3 N.sub.4 + AlN                                                                 TiB.sub.2 + ZrSi                                                                       ZrO.sub.2, Al.sub.2 O.sub.3                                                           TiO.sub.2, ZrO.sub.2, B.sub.2 O.sub.3,                                         SiO.sub.2                                           199 Si.sub.3 N.sub.4 + ZrO.sub.2                                                           ZrB.sub.2  + ZrSi                                                                      ZrO.sub.2                                                                             ZrO.sub.2, B.sub.2 O.sub.3, SiO.sub.2               200 Si.sub.2 N.sub.2 O                                                                     ZrSi + B.sub.4 C                                                                       ZrO.sub.2                                                                             ZrO.sub.2, SiO.sub.2, B.sub.2 O.sub.3, SiC          201 Si.sub.3 N.sub.4 + TiCN                                                                TiB.sub.2 + ZrSi                                                                       ZrO.sub.2, Al.sub.2 O.sub.3                                                           TiO.sub.2, ZrO.sub.2, B.sub.2 O.sub.3,                                         SiO.sub.2                                           202 Si.sub.3 N.sub.4 + TiN                                                                 ZrSi    ZrO.sub.2                                                                             ZrO.sub.2, SiO.sub.2                                203 Si.sub.3 N.sub.4 + Mo                                                                  ZrB.sub.2 + ZrSi                                                                       ZrO.sub.2                                                                             ZrO.sub.2, B.sub.2 O.sub.3, SiO.sub.2               204 Si.sub.3 N.sub.4 + WC                                                                  TiB.sub.2 + ZrSi                                                                       ZrO.sub.2                                                                             TiO.sub.2, ZrO.sub.2, B.sub.2 O.sub.3,                                         SiO.sub.2                                           205 Sialon + WC                                                                            ZrSi    ZrO.sub.2                                                                             ZrO.sub.2, SiO.sub.2, B.sub.2 O.sub.3, SiC          206 BN      ZrB.sub.2  + ZrSi                                                                      ZrO.sub.2, Al.sub.2 O.sub.3                                                           ZrO.sub.2, B.sub.2 O.sub.3, SiO.sub.2               207 TiN     TiB.sub.2 + ZrSi                                                                       ZrO.sub.2                                                                             TiO.sub.2, ZrO.sub.2, B.sub.2 O.sub.3,                                         SiO.sub.2                                           208 TiC     ZrSi    ZrO.sub.2                                                                             ZrO.sub.2, SiO.sub.2, B.sub.2 O.sub.3,                                         Si.sub.3 N.sub.4                                    209 WC      ZrSi    ZrO.sub.2                                                                             ZrO.sub.2, SiO.sub.2, B.sub.2 O.sub.3,              __________________________________________________________________________                                SiC                                            

                                      TABLE 11                                     __________________________________________________________________________                 Self-   Outermost                                                              Recoverable                                                        No. Structure                                                                              Layer   Surface Layer                                                                         Recovered Product                                   __________________________________________________________________________     210 Si.sub.3 N.sub.4                                                                       ZrSi    ZrO.sub.2                                                                             ZrO.sub.2, SiO.sub.2, B.sub.2 O.sub.3, SiC          211 Si.sub.3 N.sub.4                                                                       TiB.sub.2 + ZrSi                                                                       ZrO.sub.2                                                                             TiO.sub.2, ZrO.sub.2, B.sub.2 O.sub.3,                                         SiO.sub.2                                           212 Si.sub.3 N.sub.4                                                                       TiB.sub.2 + ZrSi                                                                       ZrO.sub.2                                                                             TiO.sub.2, ZrO.sub.2, B.sub.2 O.sub.3,                                         SiO.sub.2                                           213 C/C composite                                                                          ZrSi    Zircon ZrO.sub.2, SiO.sub.2                                214 Si.sub.3 N.sub.4 + SiC                                                                 FeSi    ZrO.sub.2, Fe.sub.2 O.sub.3                                                           Fe.sub.2 O.sub.3, SiO.sub.2                         215 Si.sub.3 N.sub.4 + ZrO.sub.2                                                           ZrB.sub.2  + ZrSi                                                                      ZrO.sub.2, Al.sub.2 O.sub.3                                                           ZrO.sub.2, B.sub.2 O.sub.3, SiO.sub.2               216 Si.sub.3 N.sub.4 + AlN                                                                 ZrSi    Zircon ZrO.sub.2, SiO.sub.2                                217 Si.sub.3 N.sub.4 + SiC                                                                 TiSi    ZrO.sub.2, SiO.sub.2                                                                  TiO.sub.2, SiO.sub.2                                218 AlN     WSi     WO.sub.2, TiO.sub.2                                                                   WO.sub.2, B.sub.2 O.sub.3, SiO.sub.2                219 Mullite LaSi    La.sub.2 O.sub.3, SiO.sub.3                                                           La.sub.2 O.sub.3, B.sub.2 O.sub.3, SiO.sub.3        220 Sialon  HfSi    HfO.sub.2, Y.sub.2 O.sub.3                                                            HfO.sub.2, B.sub.2 O.sub.3, SiO.sub.3               221 C/C composite                                                                          TaSi    ZrO.sub.2 + TaO.sub.2                                                                 TaO.sub.2, B.sub.2 O.sub.3, SiO.sub.2               222 Si.sub.2 N.sub.2 O                                                                     FeSi.sub.2                                                                             ZrO.sub.2, SiO.sub.2                                                                  Fe.sub.2 O.sub.3, SiO.sub.2                         223 Sialon + ZrO.sub.2                                                                     AlSi    Al.sub.2 O.sub.3, MgO                                                                 Al.sub.2 O.sub.3, B.sub.2 O.sub.3, SiO.sub.2        224 Sialon + TiN                                                                           ZrSi    Zircon ZrO.sub.2, SiO.sub.2                                225 Sialon + SiC                                                                           ZrB.sub.2 + ZrSi                                                                       ZrO.sub.2, Al.sub.2 O.sub.3                                                           ZrO.sub.2, B.sub.2 O.sub.3, SiO.sub.2               226 Si.sub.3 N.sub.4 + TiN                                                                 TiB.sub.2 + ZrSi                                                                       ZrO.sub.2                                                                             TiO.sub.2, ZrO.sub.2, B.sub.2 O.sub.3,                                         SiO.sub.2                                           227 Si.sub.3 N.sub.4 + Mo                                                                  AlB.sub.2                                                                              Al.sub.2 O.sub.3, Y.sub.2 O.sub.3                                                     Al.sub.2 O.sub.3, B.sub.2 O.sub.3                   228 Si.sub.3 N.sub.4 + WC                                                                  TiSi    TiO.sub.2, Al.sub.2 O.sub.3                                                           TiO.sub.2, B.sub.2 O.sub.3, SiO.sub.2               __________________________________________________________________________

EXAMPLES 229-244

The surface of each structure shown in Table 12 was covered with each self-recoverable layer shown in Table 12 to a thickness of 200 μm, and each outermost surface layer such as ZrO₂ shown in Table 12 to a thickness of 200 μm by a powder type plasma spraying. The plasma spraying was performed by a method wherein plasma was generated using argon gas in a vacuum atmosphere. The self-recoverable layer and the outermost layer thus obtained were dense with a porosity of 3% or less. A crack having a width of 0.2 mm and a depth of 200 μm was given on the outermost surface of the structure. The structure was then kept for 100 hr. in the atmospheric air at 1500° C. As a result, the cracked portion was recovered by a product material generated by the reaction between the self-recoverable layer and oxygen in the atmospheric air at 1500 degrees C. Each component of the recovered portion was shown in Table 12.

                  TABLE 12                                                         ______________________________________                                                          Self-     Outermost                                                            Recoverable                                                                              Surface Recovered                                   No.   Structure  Layer     Layer   Product                                     ______________________________________                                         229   Stainless steel                                                                           ZrSi      ZrO.sub.2                                                                              ZrO.sub.2, SiO.sub.2                        230   Cast iron  TiSi      TiO.sub.2, ZrO.sub.2                                                                   TiO.sub.2, SiO.sub.2                        231   Nb.sub.3 Al                                                                               MoSi      ZrO.sub.2, SiO.sub.2                                                                   MoO, SiO.sub.2                              232   Ni alloy   FeSi      ZrO.sub.2, Fe.sub.2 O.sub.3                                                            Fe.sub.2 O.sub.3, SiO.sub.2                 233   Cr alloy   CoSi      CoO.sub.2, ZrO.sub.2                                                                   CoO.sub.2, SiO.sub.2                        234   Carbon steel                                                                              LaSi      La.sub.2 O.sub.3, ZrO.sub.2                                                            La.sub.2 O.sub.3, SiO.sub.2                 235   Cr--Ni alloy                                                                              HfSi      HfO.sub.2, ZrO.sub.2                                                                   HfO.sub.2, SiO.sub.2                        236   Tool steel TaSi      Ta.sub.2 O.sub.5, ZrO.sub.2                                                            TiO.sub.5, SiO.sub.2                        237   Al alloy   NbSi      Nb.sub.2 O.sub.5, ZrO.sub.2                                                            Nb.sub.2 O.sub.5, SiO.sub.2                 238   Ti alloy   AlSi      Al.sub.2 O.sub.3, ZrO.sub.2                                                            Al.sub.2 O.sub.3, SiO.sub.2                 239   Ni.sub.3 (Al, Ti)                                                                         NiSi      NiO.sub.2, ZrO.sub.2                                                                   NiO.sub.2, SiO.sub.2                        240   Inconel    ZrB.sub.2 + ZrSi                                                                         ZrO.sub.2                                                                              ZrO.sub.2, B.sub.2 O.sub.3, SiO.sub.2       241   Fiber-     ZrSi + MoSi                                                                              ZrO.sub.2                                                                              ZrO.sub.2, SiO.sub.2, MoO                         reinforced Al                                                            242   One-way    ZrSi      ZrO.sub.2                                                                              ZrO.sub.2, SiO.sub.2, B.sub.2 O.sub.3             eutectic alloy                                                                 Cr--TaC                                                                  243   Free-cutting                                                                              AlSi      Al.sub.2 O.sub.3                                                                       Al.sub.2 O.sub.3, SiO.sub.2                       steel                                                                    244   SiC particle                                                                              NiSi      NiO.sub.2, ZrO.sub.2                                                                   NiO.sub.2, SiO.sub.2                              dispersion-                                                                    strengthened                                                                   Ti metal                                                                 ______________________________________                                    

EXAMPLE 245

The ceramic structures, self-recoverable layers, and outermost surface layers having the compositions shown in Examples 1 to 84, 156 to 158, and 162 were fabricated by a laminating method. A ceramic powder as a source of a structure having an average particle size of 0.5 μm was dry-mixed by a mill (automatic mixer). The mixed powder was added with a specified amount of an organic solvent, being formed in a slurry, and was formed into a green sheet by a doctor blade method. Similarly, a ceramic source for a self-recoverable layer was formed into a green sheet by the doctor blade method. Moreover, a ceramic source for an outermost surface layer was formed into a green sheet by the doctor blade method. These green sheets were laminated, and then compressed with a pressure of 98 MPa in the laminating direction into a compact. The compact thus obtained was heated in an inert gas at a temperature ranging from 1400° to 2100° C. until it was densified, thus fabricating a structure covered with the self-recoverable layer (FIG. 3). A crack having a width of 0.3 mm was given on the outermost surface of the structure up to a depth of the self-recoverable layer. The structure was then kept for 100 hr. in the atmospheric air at 1500° C. As a result, the cracked portion was recovered by a product material generated by the reaction between the self-recoverable layer and oxygen in the atmospheric air at 1500° C.

EXAMPLE 246

The self-recoverable layers and the outermost surface layers shown in Examples 1 to 132 were covered by a CVD (Chemical Vapor Deposition) process. For example, a structure was disposed in a reaction chamber, and a titanium halide gas and a boron halide gas were introduced in the reaction chamber at 1000° C., to thus cover the surface of the structure with TiB₂. Similarly, by changing the component of an introduced gas and the temperature of the reaction chamber, it is possible to cover the surface of the structure with the layer having each composition shown in the above-described examples. The covering layer thus obtained was dense with a porosity of 5% or less. A crack having a width of 0.3 mm was given on the outermost surface of the structure up to a depth of the self-recoverable layer. The structure was then kept for 1000 hr. in the atmospheric air at 1200° C. As a result, the cracked portion was recovered by a product material generated by the reaction between the self-recoverable layer and oxygen in the atmospheric air at 1500 degrees C.

From this example, it is revealed that the self-recovering function of the present invention is not affected by the covering method. The covering methods include, other than the laminating method, the conventional known methods such as a covering method, printing method, PVD process, CVD process, electrophoresis method, and spraying method. The detail of the covering method is shown, for example, in "Ceramic Covering Technique" issued by General Technical Center.

EXAMPLES 247-254

To examine what effect the thickness of a covering layer exerts on the self-recovering function, tests were made for the structures shown in Example 1. The result is shown in Table 13. As is apparent from Table 13, the thickness of the self-recoverable layer is required to be at least 1/10 that of the outermost surface covering layer for sufficiently achieving the self-recovering function.

                  TABLE 13                                                         ______________________________________                                                        Thickness of                                                                               Thickness of                                                       Self-Recoverable                                                                           Outermost Recovered                                 No.   Structure                                                                               Layer       Surface Layer                                                                            Product                                   ______________________________________                                         247   Si.sub.3 N.sub.4                                                                        ZrB.sub.2 (150 μm)                                                                      ZrO.sub.2 (200 μm)                                                                    Good                                      248   Si.sub.3 N.sub.4                                                                        ZrB.sub.2 (100 μm)                                                                      ZrO.sub.2 (200 μm)                                                                    Good                                      249   Si.sub.3 N.sub.4                                                                        ZrB.sub.2 (80 μm)                                                                       ZrO.sub.2 (200 μm)                                                                    Good                                      250   Si.sub.3 N.sub.4                                                                        ZrB.sub.2 (50 μm)                                                                       ZrO.sub.2 (200 μm)                                                                    Good                                      251   Si.sub.3 N.sub.4                                                                        ZrB.sub.2 (40 μm)                                                                       ZrO.sub.2 (200 μm)                                                                    Good                                      252   Si.sub.3 N.sub.4                                                                        ZrB.sub.2 (20 μm)                                                                       ZrO.sub.2 (200 μm)                                                                    Good                                      253   Si.sub.3 N.sub.4                                                                        ZrB.sub.2 (10 μm)                                                                       ZrO.sub.2 (200 μm)                                                                    Unusable for                                                                   a long time                               254   Si.sub.3 N.sub.4                                                                        ZrB.sub.2 (5 μm)                                                                        ZrO.sub.2 (200 μm)                                                                    Unusable for                                                                   a long time                               ______________________________________                                    

EXAMPLE 255

A moving blade and a stator blade for a gas turbine shown in FIGS. 4 and 5 were fabricated using the members obtained in Examples 1, 16, 85, and 156 to 163. A gas turbine of a class of 1500° C., which was incorporated with the member, was operated; and periodically stopped to examine the state of surface damage. There was no abnormality until 1000 hr, and cracks were generated after 2000 hr; however, the cracked portion was self-recovered by a compound produced from the self-recoverable layer. Even after 5000 hr, the cracks were buried by the self-recoverable layer, thereby preventing the oxidization of the structure as the base material.

EXAMPLE 256

A structural member, in which the surface of a Si₃ N₄ structure was covered with AlB₂ as a self-recoverable layer and sialon was covered on the outermost surface, was applied to a roll shaft used for continuous hot dip zinc plating. FIG. 6 shows a continuous hot dip zinc plating apparatus. The member was used for a roll shaft 8 and a support roll 10 in FIG. 6. A C/C composite was used for a bearing 9. The apparatus in this example was used for continuous plating in a zinc bath at 500° C. The sialon layer was worn after 1000 hr; however, the damaged portion was recovered by the intermetalic compound of Al--Zn produced by the reaction between AlB₂ as the self-recoverable layer and zinc.

The service life of a conventional steel roll shaft and bearing due to wear was three days. However, the member in this embodiment did not suffer from abnormal wear even after continuous operation for three months or more, contributing to the uniform plating work.

As shown in this example, a product material generated by the reaction between molten metal in a service environment and a self-recoverable layer can achieve the recovering function.

EXAMPLE 257

A structural member, in which the surface of a Si₃ N₄ structure was covered with NbB₂ as a self-recoverable layer and sialon was covered on the outermost surface, was applied to a roll shaft for continuous hot dip aluminum plating. The member was used for a roll shaft 8 and a support roll 10 in FIG. 6. A C/C composite was used for a bearing 9. The apparatus in this example was used for continuous plating in an aluminum bath at 680° C. The sialon layer was worn after 1000 hr; however, the damaged portion was recovered by the intermetalic compound of Nb--Al produced by the reaction between NbB₂ as the self-recoverable layer and aluminum. The service life of a conventional steel roll shaft and bearing was four days. However, the member in this embodiment did not suffer from abnormal wear even after continuous operation for three months or more, contributing to the uniform plating work.

EXAMPLE 258

A metal Si powder having an average particle size of 1 μm (100 parts by weight) was added with a mixture of polyethylene, stearic acid and synthetic wax (13 parts by weight), and then kneaded, to form a source as a ceramic part. The source was extruded into a plate having a size of 100×100×10 mm by an extruder. A bundle (diameter: 1.5 mm) of carbon fibers (diameter: 10 μm, length: 1000 μm) was molded on this plate, to form a resistance measuring circuit for life diagnosis.

The compact was heated up to 1350° C. with a temperature rising rate of 1° C./min, and cooled in a furnace, to obtain a Si₃ N₄ reaction-sintered ceramic having a resistance measuring circuit for life diagnosis. The degreasing for a binder was performed in the temperature rising process. The surface layer portion of the reaction-sintered ceramic thus obtained was covered with ZrB₂ as a self-recoverable layer to a thickness of 200 μm by plasma spraying. A dense film of ZrO₂ was formed thereon to a thickness of 200 μm for ensuring the heat resistance by plasma spraying. In the reaction-sintered body, since the surface layer portion was porous and ZrB₂ was produced in voids, a film with a high bonding strength could be obtained. A copper wiring was brazed to the end portion of the inner circuit, which was connected to an external measuring apparatus.

The ceramic part was applied to a lining part in a combustor for a 1500° C. class gas turbine. As a result, in the ceramic part, when the ZrO₂ film was broken by a combustion test, ZrB₂ was simultaneously oxidized to recover the ZrO₂ film. At the same time, the resistance of the resistance measuring circuit for life diagnosis changed. The changed resistance was compared with a threshold for performing life diagnosis for the lining part in the combustor (FIG. 7).

Similarly, a complex ceramic (Si₃ N₄ /Si₂ N₂ O, Si₃ N₄ /AlN, Si₃ N₄ /Al₂ O₃, Si₃ N₄ /ZrO₂, Si₃ N₄ /mullite, Si₃ N₄ /SiC or the like) having a resistance measuring circuit for life diagnosis was fabricated by the reaction sintering method using the above Si powder mixed with powders or whiskers of Si₃ N₄, Si₂ N₂ O, AlN, Al₂ O₃, ZrO₂, B₄ C, BN, mullite and the like. The complex ceramic thus obtained was applied to the lining part in the combustor, which gave the same result as the above-described ceramic part, that is, which achieved the life diagnosis by the excellent self-recovering function.

The formation of the ceramics used in the present invention may be performed by methods of doctor blade, die compacting, CIP, injecting molding, slip-casting, extrusion and the like according to the shapes of the parts.

As the binder for molding the above-described green sheet, there may be used known binders for molding ceramics, which include high polymer materials such as polyvinyl butyryl and polyethylene, silicone-family compounds, and polysilane-family compounds. The degreasing method for these binders is not limited, but it is preferably performed by controlling the temperature rising rate during sintering.

EXAMPLE 259

Powders of ZrO₂ (50 parts by weight), SiO₂ (30 parts by weight), Al₂ O₃ (20 parts by weight) were mixed with triethanol amine (1 part by weight) and distilled water (65 parts by weight), to form a source for a ceramic part. Using the source thus obtained, there were fabricated, by slip casting, a tonguedish for molten metal, a refractory for mold inlet, and a mold. In molding, a bundle of carbon fibers (diameter: 100 μm, length: 50 cm, number: 100) were inserted in each member, to form a resistance measuring circuit for life diagnosis.

The above molded product was heated up to 1650° C. with a temperature rising rate of 5° C./min in an oxidizing atmosphere, being kept for 2 hr, and was cooled in a furnace, thus obtaining a ceramic having a resistance measuring circuit for life diagnosis. The degreasing for a binder was performed in the temperature rising process. The end portion of the life diagnosis circuit was connected to an external circuit by way of a copper wiring. Moreover, the life diagnosis circuit was insulated by an Al₂ O₃ protecting tube.

The surface layer portion of the ceramic thus obtained was covered with LaB₂ as a self-recoverable layer by plasma spraying, and a dense film of sialon was formed thereon to a thickness of 200 μm for ensuring a heat resistance by plasma spraying.

The ceramic parts were applied to an aluminum continuous casting machine. As a result of the testing, when each ceramic part was worn out and cracked, the cracked portion was recovered by a LaAl layer and simultaneously the resistance was changed. The change in the resistance was detected, to give the life of the part to an operator. This makes it possible to effectively perform the maintenance of the continuous casting machine.

EXAMPLE 260

An outer wall tile and nose cone for a space shuttle were fabricated using the inventive members obtained in Examples. 85, 94 and 95. The members were subjected to a burner test at 2000° C., and the equipment was periodically stopped to examine the damaged state on the surface. There was no abnormality until 50 hr, and cracks were generated at the ZrO₂ layer after 100 hr; however, the cracked portion was recovered by a product material generated from the self-recoverable layer, thus preventing the oxidization of the structure as the base material.

EXAMPLE 261

Using oxides produced from the self-recoverable layers, other than those shown in the above-described examples, have also the self-recovering function. For example, when the oxides shown in Table 14 were used in environments having temperature lower than the melting points thereof, there could be obtained the stable recovered portions. In particular, a Al₂ O₃ system and MgO system are stable. As the self-recoverable layers, borides and silicates capable of producing the compounds shown in Table 14 by oxidization may be selected.

                  TABLE 14                                                         ______________________________________                                         Recovered Product from Self-                                                                    Recovered Product from Self-                                  Recoverable Layer and Melting                                                                   Recoverable Layer and Melting Point                           Point (°C.)                                                                              (°C.)                                                  ______________________________________                                         Al.sub.2 O.sub.3 --BaO                                                                    1425      Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2                                                       1400                                      Al.sub.2 O.sub.3 --BeO                                                                    1835      K.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2                                                        1686                                      Al.sub.2 O.sub.3 --CaO                                                                    1400      MgO--Al.sub.2 O.sub.3 --SiO.sub.2                                                              1470                                      Al.sub.2 O.sub.3 --CeO.sub.2                                                              1800      CaO--Al.sub.2 O.sub.3 --SiO.sub.2                                                              1553                                      Al.sub.2 O.sub.3 --FeO                                                                    1310      BaO--Al.sub.2 O.sub.3 --SiO.sub.2                                                              1750                                      Al.sub.2 O.sub.3 --MgO                                                                    1995      FeO--Al.sub.2 O.sub.3 --SiO.sub.2                                                              1560                                      Al.sub.2 O.sub.3 --MnO                                                                    1520      CaO--MgO--SiO.sub.2                                                                            1500                                      Al.sub.2 O.sub.3 --Na.sub.2 O                                                             1410      CaO--TiO.sub.2 --SiO.sub.2                                                                     1352                                      Al.sub.2 O.sub.3 --SiO.sub.2                                                              1595      BaO--TiO.sub.2 --SiO.sub.2                                                                     1400                                      Al.sub.2 O.sub.3 --TiO.sub.2                                                              1705      CaO--ZrO.sub.2 --SiO.sub.2                                                                     1453                                      Al.sub.2 O.sub.3 --ZnO                                                                    1700      β-Spodumene                                                                               1000                                      Al.sub.2 O.sub.3 --ZrO.sub.2                                                              1885      Barium Osumilite                                                                               1250                                      MgO--CaO   2370      Mullite         1500                                      MgO--SiO.sub.2                                                                            1543      Hexacelsian     1700                                      MgO--TiO.sub.2                                                                            1583      MgO--GeO.sub.2  1099                                      MgO--ZrO.sub.2                                                                            1500      ZrO.sub.2 --SiO.sub.2                                                                          1687                                      ZrO.sub.2 --TiO.sub.2                                                                     1760      ZrO.sub.2 --Nb.sub.2 O.sub.5                                                                   1430                                      SiO.sub.2 --BaO                                                                           1370      SiO.sub.2 --CaO 1436                                      SiO.sub.2 --SrO                                                                           1358      SiO.sub.2 --TiO.sub.2                                                                          1550                                      SiO.sub.2 --ZnO                                                                           1432      TiO.sub.2 --BaO 1317                                      ______________________________________                                    

Moreover, the compounds shown in Table 14 may be used as they are. In this case, the environmental temperature must be lower than the melting points of the self-recoverable layers.

A barrier may be inserted between the surface of a structure of a ceramic or C/C composite and a self-recoverable layer, as needed.

According to further embodiments of the invention the self-recoverable layer of the present invention can be formed into a complex shape, and therefore, it is easily applied to parts required for high reliability (for example, engine parts, parts for a continuous metal plating apparatus, first wall parts of a nuclear fusion reactor in an atomic power plant, moving blade and stator blade of a gas turbine, parts for combustor, shroud parts and the like). 

What is claimed is:
 1. A process for making a self-recovering product for use in a reactive environment, said process comprising:forming a base member comprising at least one kind of ceramics, metals and C/C composites; and forming a self-recovering protective layer substantially isolating said base member and exposed to a reactive environment, wherein said self-recovering protective layer comprises: an outer layer of protective material comprising an oxide compound resistive to the reactive environment; and an inner layer of protective material, located between the base member and the outer layer of protective material, said inner layer comprising a reactive material including a boride selected from the group consisting of Aluminum, Hafnium, Tantalum, Yttrium and Zirconium, wherein said reactive material is present in quantities sufficient to substantially reisolate the base member, by means of reaction between the reactive material and the reactive environment, when the reactive material is exposed to the reactive environment through an opening in the outer layer of protective material.
 2. A process as in claim 1 wherein a difference in a thermal expansion coefficient between said reactive material and said protective material is within a range of 4×10⁻⁶ /°C. or less.
 3. A process as in claim 1 wherein said self-recovering protective layer comprises a distribution of a protective material and a reactive material, wherein said protective material is in higher concentrations near the reactive environment and said reactive material is in higher concentrations near the base member.
 4. A process as in claim 1 wherein said self-recovering protective layer comprises an even distribution of said protective material and said reactive material.
 5. A process as in claim 1 wherein said inner layer of protective material comprises a silicate selected from the group consisting of Aluminum, Hafnium, Tantalum, Yttrium and Zirconium.
 6. A process as in claim 1 wherein said reactive material comprises a tantalum compound.
 7. A process as in claim 1 wherein said protective material has a vapor pressure of 1×10⁻⁴ N/m² or less in the atmospheric air at 1500° C.
 8. A process as in claim 7 wherein said protective material comprises a zirconium compound.
 9. A process as in claim 7 wherein said protective material comprises an aluminum compound.
 10. A process as in claim 7 wherein said protective material comprises an yttrium compound.
 11. A process as in claim 7 wherein said protective material comprises a tantalum compound.
 12. A process as in claim 7 wherein said protective material comprises a hafnium compound.
 13. A process as in claim 1 wherein a difference in a thermal expansion coefficient between said reactive material and said protective material is within a range of 4×10⁻⁶ /°C. or less.
 14. A process as in claim 1 wherein said reactive material has a vapor pressure of 1×10⁻⁴ N/m² or less in the atmospheric air at 1500° C.
 15. A process as in claim 14 wherein said reactive material comprises a zirconium compound.
 16. A process as in claim 14 wherein said reactive material comprises a silicon compound.
 17. A process as in claim 14 wherein said reactive material comprises an aluminum compound.
 18. A process as in claim 1 wherein said reactive material comprises a silicon compound, said silicon compound further comprising at least one or more elements from a group consisting of a silicate of Zr, Hf, Ta, Al and Y, and which is added in an amount of about 30 vol % or more.
 19. A process as in claim 1 wherein said reactive material comprises a boron compound, said boron compound further comprising at least one or more elements from a group consisting of borides of Zr, Hf, Ta, Al and Y, and which is added in an amount of about 30 vol % or more.
 20. A process as in claim 1 wherein said protective material comprises an oxide of one or more elements selected from a group consisting of Zr, Hf, Ta, Al and Y. 